The present invention relates to lock washers, and specifically to conductive lock washers for use in a disk drive or similar apparatus.
Present day high capacity disk drives produce vibrations, such as created when the device is operated in upper and lower dynamic rocking modes. Such vibrations can cause the drive to malfunction. Thus, there is a need to damp the vibrations to ensure optimal drive performance. This can be done by securing the spindle motor shaft to a spindle washer damper (SWD), which is connected to a top cover, which in turn is connected to a baseplate. The SWD acts to attenuate vibrations produced within the drive thereby facilitating optimal drive performance.
The SWD is typically attached to the top cover of the disk drive by a non-conductive adhesive. However, the non-conductive adhesive can create an electrostatic charge problem. As electrostatic charge builds up on the spinning disk surfaces, the non-conductive adhesive creates a barrier that prevents the charge from being dissipated through the cover and baseplate. If this charge cannot be dissipated, voltage can build up across the head/disk gap until the electrostatic charge discharges across the gap, potentially causing damage to the disk surface and head, and further potentially causing data loss.
Additionally, as the SWD is attached to the top cover by an adhesive, torque generated by tightening the spindle motor shaft screw against the SWD and spindle can be transferred to the SWD and the adhesive. This can result in the SWD being twisted off of the top cover. In turn, a loose SWD can cause decreased performance or even failure of the disk drive, or if the failure occurs during the manufacture and assembly, it can result in costly reworks of the drive.
Therefore, there is a need for a conductive path from the disk surface, through the spindle motor shaft to the spindle washer damper and to the top cover. Additionally, it would be advantageous to have a torque transferring feature to help prevent the spindle motor shaft screw from applying a torque directly to the SWD and twisting it off of the adhesive and top cover during and after tightening. It would also be advantageous to have a torque locking feature, to prevent the spindle motor shaft screw from loosening during operation of the drive.
Others in the industry have developed ways to provide the ground path. For example, one method is to press the spindle motor shaft into the baseplate creating interference coupling which is able to dissipate any electrostatic charge to the baseplate. However, this method has the disadvantage of reduced precision in the manufacturing process, reducing spindle to baseplate alignment and potentially degrading drive performance. Others have also used a conductive adhesive to connect the shaft to the baseplate. However, this conductive adhesive is a potential source of out gassing and contamination failures. Additionally, this method does not transfer torque away from the SWD which can result in the previously mentioned manufacturing reworks caused by twisting the SWD off of its adhesive connection to the top cover. This method also allows the spindle motor shaft to work its way loose, as there is no torque locking between the spindle motor shaft and the SWD.
Others have also used a conductive washer placed between the spindle motor shaft screw and the SWD such that the washer also contacts the top cover to provide a conductive path. As depicted in FIG. 8, this conductive washer 20 has three outer tabs 21 that are positioned between the top cover and SWD. The nine inner tabs 22 contact the under surface of the spindle motor shaft screw head as it is tightened into the spindle shaft. However, this method does not reduce or eliminate the application of a torque on the SWD resulting in the possible disengagement of the SWD from the adhesive connecting it to the top cover, nor does it provide a locking feature to prevent loosening of the spindle motor shaft screw.
Accordingly, there is a need for an apparatus and method that overcomes all of the problems described above.
The present invention is designed to overcome the aforementioned problems and meet the above-described needs.
The present invention provides a method and apparatus for providing a conductive path between the disk and the top cover through the spindle washer damper. More specifically, in one embodiment, the lock washer is advantageously formed such that upon insertion and tightening of the spindle motor shaft screw into the spindle, the washer deforms and makes a conductive contact with the top cover, the spindle motor shaft screw and spindle washer damper. Accordingly, electrostatic discharge of voltage built up on the spinning disks can be discharged through the conductive path without a potential discharge across the head/disk gap.
Additionally, the lock washer is advantageously formed from a series of alternating inner tabs and outer tabs interconnected by a thin piece of resilient metal. The lock washer also has a curved cross-section, such that the only portions of the lock washer in contact with the spindle washer damper, prior to the complete tightening of the spindle motor shaft screw, are the thin pieces of resilient metal which connect the inner tabs to the outer tabs. This configuration acts to reduce hoop stresses required to deform the washer, thereby reducing the torque applied to the spindle washer damper. More specifically, as the spindle motor shaft screw is being tightened the outer tabs lift and come into contact with the under surface of the top cover. Because the outer tabs contact with the top cover at a larger radial distance from the center of the lock washer than where the thin pieces of resilient metal contact the spindle washer damper and because the outer tabs place more surface area in contact with the top cover than the thin resilient strips place in contact with the spindle washer damper, more of the torque generated by the tightening of the spindle motor shaft screw is transferred to the top cover rather than to the spindle washer damper.
Furthermore, in one embodiment, the curved cross-section of the lock washer is advantageously formed to allow the lock washer to be placed between the top cover and the spindle washer damper such that it does not apply any pressure between the two members prior to the insertion of the spindle motor shaft screw, and deforms upon the insertion of the screw into the spindle shaft to contact the spindle washer damper, the top cover and the screw. In addition, upon insertion of the spindle motor shaft screw into the spindle shaft, the lock washer continues to apply lifting pressure to the under side of the screw head, thereby resisting loosening of the screw during operation of the disk drive.